Industrial radiant heater

ABSTRACT

High temperature industrial radiant heaters may be improved by coating a refractory or lining in and adjacent to the flame impingement zone which refractory or lining does not transmit fuel to the burner with a metal oxide catalyst or metal oxide catalyst precursor other than iron, iron oxides or mixtures thereof, to promote the burning of one or more of the fuel, and combustion products thereof.

FIELD OF THE INVENTION

The present invention relates to burners for fluid fuels and inparticular burners for volatile organic fuels such as natural gas or afuel rich in natural gas. More particularly the present inventionrelates to heaters having improved radiant efficiency and which burncleaner.

BACKGROUND OF THE INVENTION

There is a significant field of art relating to non-flame catalyticcombustion. In this type of process a fuel to be burned passes over asupported catalyst at an elevated temperature and the fuel is consumed.The most common of this type of catalytic burner is the catalyticconverter in cars. The support is typically in the shape of an openhoneycomb and the exhaust gasses pass through the honeycomb wherecombustion gasses are consumed or converted to more environmentallyacceptable products. The converter technology does not use flameimpingement of the present technology.

German Patent DE 195 32 152 B4 discloses the use of a coating of metaloxide based catalysts such as FeO, Fe₂O₃, and Fe₃O₄ on fireproof(refractory) materials placed on the walls of a residential fireplaceand/or chimney. The catalyst help burn the “off gas” from the solid fuelburning in fire. The reference does not teach or suggest a flameimpingement process as required by the present invention. Further thereference teaches iron oxide catalysts which are not included in thescope of the present invention. U.S. Pat. No. 3,565,830 issued Feb. 23,1971 to Keith et al., assigned to Englehard Minerals and ChemicalsCorporation discloses a catalytic converter comprising a honeycombsupport having a porosity of at least 0.1 to 0.3 cc/g upon which isdeposited a platinum group metal. The converter differs from the heaterof the present invention in that it does not require a direct flameimpingement.

U.S. Pat. No. 6,431,856 issued Aug. 13, 2002, to Maenishi et al. teachesa refractory supporting one or more combustion catalysts in a combustionchamber. However, the refractory is shaped in such a manner that thefuel gas or combustions products thereof pass through the refractory.The present invention requires a refractory or lining through whichneither the fuel nor the combustion products pass.

The present invention seeks to provide a simple method to improve theheating and particularly the radiant heating of a heater. Moreparticularly the present invention seeks to provide a heater, andparticularly a high temperature heater, having a higher wall temperaturein or adjacent a flame impingement zone and a higher emisvity(radiation) under equivalent fuel and combustion air input.

SUMMARY OF THE INVENTION

The present invention provides in a fluid fueled heater, comprising amechanical burner, a flame impingement zone adjacent said burner, and arefractory or lining in and adjacent to the flame impingement zone whichrefractory or lining does not transmit fuel to the burner theimprovement of substantially coating the refractory or lining with ametal oxide catalyst or metal oxide catalyst precursor other than iron,iron oxides and mixtures thereof (e.g. the catalyst or precursor can notbe Fe, FeO, Fe₂O₃, Fe₃O₄ or mixtures thereof) to promote the burning ofone or more of the fuel, and combustion products from the fuel.

The present invention further provides a process for preparing arefractory or lining having a metal oxide or metal oxide precursorcomponent other than iron oxides, and mixtures thereof, which catalysesthe burning of a fluid fuel or combustion products from a fluid fuelwhen subject to flame impingement selected from the group consisting of:

a) direct application of a coating composition comprising said metaloxide or metal oxide precursor to the refractory or lining surface; and

b) incorporating said metal oxide or metal oxide precursor into therefractory or lining composition during manufacture of said refractory.

The present invention further provides a method to increase burnerstability by substantially coating the refractory or lining in the flameimpingement zone with a metal oxide catalyst or precursor for a metaloxide catalyst other than iron, iron oxides and mixtures thereof, topromote the burning of one or more of the fuel and the combustionproducts.

BRIEF DESCRIPTON OF THE DRAWINGS

FIG. 1 is a schematic drawing of a combustion testing apparatus (CTA)used in the Examples.

DETAILED DESCRIPTION

As used in this specification “substantially coating” means coating therefractory or lining in the flame impingement zone.

The heaters of the present invention may be used in a number ofapplications typically industrial applications where process streams areheated while passing through metallic tubes in an oven, furnace orradiant heater including the hot box of a steam cracker. Generally, thepresent invention is useful in high temperature burner applicationshaving a flame impingement zone. Additionally the present inventionprovides improved flame/burner stability.

The refractory material may be any type of refractory materials that arecommonly used in the construction of a furnace refractory wall. Examplesof such refractory materials include dolomites, silicon carbide,aluminates (Al₂O₃), aluminum silicates, chromites, silica, alumina,zirconia (ZrO₂), and mixtures thereof, preferably silica, alumina(Al₂O₃), aluminum silicates, zirconia, (ZrO₂), and mixtures thereof.Such a refractory may optionally be non-porous in nature, even thoughthe mentioned refractory materials are typically porous. Typically therefractory will be porous and have a porosity of not less than 0.1 cc/g.Typically the porosity may be from 0.1 to 0.5 cc/g, preferably from 0.1to 0.3 cc/g. The intended coating is to cover a two dimensionalrefractory wall surface, rather than to impregnate a significant depthof refractory material with the intended combustion catalysts. Morespecifically, it is irrelevant how deep the coated catalysts maypenetrate into the refractory wall. Generally the coating may have athickness from 0.0001 to 5 mm thick, typically 0.0001 to 1 mm thick. Therefractory could take any convenient form such as bricks, plates andslabs.

The present invention also contemplates a lining rather than or inaddition to the refractory. The lining needs to be suitable for thetemperature at which the burner is operated from 600° C. to 1300° C.,typically from 850° C. to 1300° C., preferably from 900° C. to 1300° C.The lining may be a suitable metal applied to or over the refractory.The metal could be cast or wrought iron or more typically steel such asstainless or high temperature steel. For lining on the blades of naturalgas turbines, the metal is typically nickel based alloys. The metalsurface may be attached to or encompass (e.g. folded over) a refractorysubstrate. The metal would protect the refractory from ablative losses.

The catalyst may be selected from the group oxides of perovskites,hexaaluminate, spinels, PtO, PdO, MgO, Ce₂O₃, CoO, Cr₂O₃, CuO, MnO, NiO,and their precursors and mixtures thereof. The catalyst does not includeiron, iron oxides, and mixtures thereof or their precursors. Preferablythe catalyst is selected from the group consisting of PtO, PdO, MgO,Ce₂O₃, CoO, Cr₂O₃, CuO, MnO, NiO and mixtures thereof. Most preferablythe catalyst is selected from the group consisting of PtO, PdO, Ce₂O₃,CoO, Cr₂O₃, CuO, MnO, NiO and mixtures thereof.

The catalyst may be prepared as an emulsion, solution or a paste (“mud”)and applied to the refractory or lining in any convenient manner such aspainting, spraying or roller. The coating could be applied to therefractory prior to incorporation into the heater or after incorporationinto the heater for example during routine maintenance or during a turnaround. The catalyst may be, incorporated into the refractory or theliner during manufacture. Although, as noted above, the affect isprimarily a surface affect and the catalyst need not be incorporatedinto the interior of the refractory or liner.

The fuel should be fluid. Preferably the fuel is a gaseous hydrocarbonsuch as natural gas or a natural gas rich (e.g. greater than 30%(volume) of natural gas) fuel. However, the fuel could also be liquidsuch as naphtha stream, a gas oil or a vacuum gas oil, and the like.

As noted above the refractor or liner is preferably exposed to theburner flame or at least a portion of the burner flame in the flameimpingement zone. There are several improvements which may be achievedby the present invention. There is a more complete combustion of thefuel and in particular a more complete combustion of the initialcombustion products from the fuel. There is also a reduction in carbonmonoxide and NO_(x) in the exhaust gases

In the experiments a combustion testing apparatus (CTA) was used. FIG. 1is a schematic sectional drawing showing the apparatus in which likeparts are designated with like numbers. Forced air 1 and natural gas 2,pass valves 3, enter the mass flow controller 5 under controlledpressures shown on the gauges 4 and then flow into a laboratory type ofgas burner 6. In addition to the forced air 1, there is a fixed amountof draft air flowing through the opening 16 at the bottom of thecombustion chamber in which the burner 6 is inserted. A flame 7 is firedagainst the surface of a refractory brick 8 which is backed by anelectric block heater 9 in order to prevent severe heat loss from thebrick and to better control the brick surface temperature. During theexperiments, five spots on the surface of a test refractory brick (withor without catalyst coating) are selected for temperature measurement bya laser pyrometer 10, during which the insolated window cover 11 isopened. Additionally, the temperature of the combustion chamber 12 andtemperature of the exhaust gas 13 are also monitored through type-Kthermocouples. The combustion chamber was insulated with ceramic fibreinsulation materials 14. The exhaust gas leaving the combustion chamber15 enters into a centre ventilation system.

Table 1 lists two sets of parameters for both fuel rich flame (FRF) andnormal flame (NF) conditions. TABLE 1 Typical Conditions Employed by CTAfor Combustion Tests Parameter Value Fuel Rich Flame Normal Flame TestParameter (FRF) (NF) Natural gas flow rate (slpm) 20 12 Flow rate of aforced air 55 55 (slpm) Flow rate of draft air (slpm) ˜82.4 ˜82.4 Blockheater temperature 700 700 (° C.) Size of the fire brick 6″ × 9″ × 1.5″6″ × 9″ × 1.5″ Firebrick material Al₂O₃: 67 wt %, SiO₂, 30.5 wt %, +other metal oxides in trace amount

EXAMPLE 1

Two commercially available combustion catalysts, made of non-noblemetals and intended for use at temperature up to 1200° C., were obtainedfrom catalyst providers. Each catalyst was provided in an undisclosedslurry form and brush-coated on the surface of a firebrick. The coatedcatalysts were then left to dry under ambient conditions for 24 hoursbefore the coated bricks were heat-treated in a muffle oven to calcinethe catalyst. These two coated bricks, plus an un-coated firebrick asreference, were tested using the CTA and under the test conditionsspecified in Table 1. The test results are given in Table 2. TABLE 2Comparative Results of Catalyst Coated Bricks and an Un-Coated Brick NFTesting Condition FRF Testing Condition Un- Un- Coated Coated Catalyst ACatalyst B Firebrick Catalyst A Catalyst B Firebrick Average Brick 940.3942.9 930.8 847.1 809.0 788.5 Temperature (° C.) Average 0.62 0.60 0.570.60 0.59 0.58 Brick Emissivity Combustion 437.7 421.6 412.2 678.5 656.6641.9 Chamber Temperature (° C.) Exhaust Gas 332.7 334.8 302.7 554.4567.1 524.9 Temperature (° C.)

Example 2

Five nitrates salts (Mg(NO₃)₂, Ce(NO₃), Co(NO₃)₂, Cu(NO₃)₂, andMn(NO₃)₂) were dissolved in 200 ml of de-ionized water each to preparefive nitrate solutions of a nominal 5 weight % in concentration. A spraybottle was used to contain one of these solutions at a time and to spraycoat the nitrate solution on a fresh refractory brick surface. Eachbrick surface, albeit not controlled exactly, was sprayed with onesolution for 30 times. The wet bricks were then left in a laboratoryfume hood at ambient conditions to dry for about 15 hours. After thedrying, each of these five coated bricks was put in a muffle oven atroom temperature, which was heated at about 10° C./min from ambienttemperature to 600° C. and held at 600° C. for 30 minutes before coolingdown. After a complete cooling to ambient temperature, these bricks weretested under the same testing conditions as used for Example 2. The heattreatment procedure was developed based on a set of separate experimentsusing a thermal balance, which confirmed that this heat treatmentprocedure will convert the nitrates into their corresponding oxides.More precisely, these oxides are primarily MgO, Ce₂O₃, CoO, CuO and MnO,respectively.

Using again the CTA, these five coated bricks were tested under both NFand FRF conditions as given in Example 2. However, it is worthmentioning that due to some modifications made to the CTA (the spacingbetween the burner and the firebrick) before this batch of tests, theseresults should not be compared with the results given in Example 2.Nevertheless, the results in Table 3 were obtained under the identicalconditions and therefore can be compared. TABLE 3 Comparative Results ofMetal Oxides Coated Bricks and an Un-Coated Brick Average Brick ChamberExhaust Gas Average Temperature Temperature Temperature Brick (° C.) (°C.) (° C.) Emissivity NF Testing Condition Un-Coated 921.4 440.0 362.90.58 Firebrick MgO 933.5 450.1 369.6 0.64 Ce₂O₃ 924.4 438.9 373.7 0.68CoO 924.8 455.3 370.0 0.70 CuO 908.5 434.0 371.0 0.75 MnO, 912.3 448.3381.3 0.69 FRF Testing Condition Un-Coated 820.5 674.1 541.1 0.59Firebrick MgO 835.8 723.9 572.6 0.62 Ce₂O₃ 835.3 657.5 560 0.66 CoO849.8 680.6 561.4 0.68 CuO 850.3 648.2 566.3 0.70 MnO, 828.3 650.7 568.40.66

These results confirm that there are noticeable improvements in themeasured temperatures for all five oxides under the FRF condition. Forexample, the maximum increase in brick surface temperature is about 30°C. with CuO while the maximum rise in chamber temperature was observedfrom MgO for about 50° C., which shows also a maximum increase inexhaust temperature for about 32° C. In contrast, these observedimprovements become less significant under NF test condition. Forexample, the highest increase in surface temperature is about 12° C.from the MgO coated brick, whilst decreases in surface temperature werealso observed with the CuO and MnO coated bricks. However, from bothtest conditions, the emissivity with the coated bricks are seen toimprove, possibly due to the changed surface compositions.

Example 4

In order to evaluate the potential benefits from using the abovementioned catalysts as refractory coating, an ethylene furnace simulator(SPYRO from TECHNIP PYROTEC) was used to simulate the radiant boxoperation of a SRT butane cracker which is heated by natural gas at aNOVA Chemicals' production plant. Using the real operating parameters(about 4,000 kg/hr butane feed), two cases were considered forsimulation: a base case and a modified case which assumes about 14° F.temperature rise on refractory wall. Considering the same butaneconversion in both cases, the simulation results (Table 4) show clearlythat with about 14.4° F. (8° C.) temperature rise on refractorytemperature, the overall efficiency in radiant box increases by 0.98%.As a result, a slightly less firing is required, suggesting a possiblesaving in fuel gas. Furthermore, with the refractory wall temperatureincreasing, a slight increase in coil outlet temperature could also berealized. Considering together that the maximum tube skin temperature islowered by 7.5° F. (about 4° C.) than in the base case, these results dosuggest that the radiant box become more homogenous in terms oftemperature. TABLE 4 Spyro Simulation Results of a SRT Butane CrackerBase Modified Case Case Change Feed Flow Per Coil (Lb/H) 8666.8 8666.80.0 Coil Inlet Temperature (° F.) 1040 1040 0.0 Average RadiantRefractory 2106 2120.4 +14.4 Temperature (° F.) Radiant Box EfficiencyOn Fired 39.029 40.008 +0.98 Heat (%) Fired Heat Based On LHV 186.7182.8 −3.9 (MBTU/H) Coil Outlet Temperature (° F.) 1580 1583.4 +3.4Maximum Tube Skin Temperature 1894.4 1886.9 −7.5 (° F.)

1. In a fluid fueled heater, comprising a mechanical burner, a flameimpingement zone adjacent said burner, and a refractory or lining in andadjacent to the flame impingement zone which refractory or lining doesnot transmit fuel to the burner the improvement of substantially coatingthe refractory or lining with a metal oxide catalyst or metal oxidecatalyst precursor other than iron, iron oxides, and mixtures thereof,to promote the burning of one or more of the fuel, and combustionproducts from the fuel.
 2. The heater according to claim1, wherein thefuel is fluid hydrocarbon.
 3. The heater according to claim 2, whereinthe refractory is selected from the group consisting of dolomites,silicon carbide, aluminates, aluminum silicates, chromites, silica,alumina, zirconia, magnesia, Al₂O₃, and ZrO₂ and mixtures thereof. 4.The heater according to claim 3, wherein during operation the refractoryis at a temperature from 600° C. to 1300° C.
 5. The heater according toclaim 4, wherein the refractory has a porosity of not less than 0.1cc/g.
 6. The heater according to claim 5, wherein the refractory orlining is in the form of bricks, a metal lining on refractory bricks orrefractory metals.
 7. The heater according to claim 6, wherein the fuelis a gaseous hydrocarbon.
 8. The heater according to claim 7, whereinthe metal oxide is selected from the group consisting of oxides ofperovskites, hexaaluminate, spinels, PtO, PdO, MgO, Ce₂O₃, CoO, Cr₂O₃,CuO, MnO, NiO, and their precursors and mixtures thereof.
 9. The heateraccording to claim 8, wherein the thickness of the metal oxide coatingon the refractory from 0.0001 to 5 mm thick.
 10. The heater according toclaim 9, wherein the refractory is selected from the group consisting ofsilica, Al₂O₃, aluminium silicates, ZrO₂, magnesia and mixtures thereof11. The heater according to claim 10, wherein the metal oxide isselected from the group consisting of oxides of PtO, PdO, MgO, Ce₂O₃,CoO, CuO, Cr₂O₃, MnO, NiO, and a mixture thereof
 12. The heateraccording to claim 11, where in a refractory is exposed to the flame inthe flame impingement zone.
 13. The heater according to claim 11,wherein a metal liner is exposed to the flame in the flame impingementzone.
 14. The heater according to claim 12 which is in the radiantheater of a steam cracker.
 15. The heater according to claim 14, whereinthe fuel is natural gas or a natural gas rich fuel.
 16. The heateraccording to claim 13 which is in the radiant heater of a steam cracker.17. The heater according to claim 16, wherein the fuel is natural gas.18. A process for preparing a refractory or lining having a metal oxideor metal oxide precursor component which catalyses the burning of afluid fuel or combustion products from a fluid fuel when subject toflame impingement selected from the group consisting of: a) directapplication of a coating composition comprising said metal oxide ormetal oxide precursor to the refractory or lining surface; and b)incorporating said metal oxide or metal oxide precursor into therefractory or lining composition during manufacture of said refractory.19. The process according to claim 18, wherein said metal oxide or metaloxide precursor is applied to the surface of said refractory or liningin the form of a paint or paste.
 20. The process according to clam 19,wherein said paint is applied to the surface of said refractory orlining by an applicator selected from the group consisting of brush,roller and spray device.
 21. The process according to claim 20 which iscarried out prior to installation of said refractory or lining in saidburner.
 22. The process according to claim 20, which is carried outduring scheduled maintenance of said burner.
 23. A method to increaseburner stability by substantially coating the refractory or lining inthe flame impingement zone with a metal oxide catalyst or precursor fora metal oxide catalyst to promote the burning of one or more of the fueland the combustion products.